I don’t think it’s about the choice or not...it pretty much means, darn it, it’s happening again. A lot of words expand In meaning over time without regard to every detail or even the main detail of the original word. For example I’m reading a book about passion, originally it meant suffering but it has since evolved to today’s meaning which is almost a 180. Anyways the root here is laps which means to slip or slide and re is just again. So I’m slipping again is the basic meaning of relapse and as such I feel it applies well to both situations.